1. Field of the Invention
The present invention relates in general to the completion of oil and gas wells and, in particular, to perforation and fracturing processes which are performed during completion operations.
2. Description of the Prior Art
In drilling operations for the production of oil and gas deposits, operators strive to maximize both the rate of flow and the overall capacity of hydrocarbon from the subsurface formation to the surface where it can be recovered. Various stimulation techniques have been developed, one of the most commercially successful techniques being referred to as “hydraulic fracturing”. The rate of flow or production of hydrocarbon from a geologic formation is naturally dependent on numerous factors. One of the most obvious of these factors is the radius of the borehole; as the radius of the borehole increases, the production rate increases, generally speaking. A related factor is the number and quality of the flow paths from the formation to the borehole available to the migrating hydrocarbon. A fracture or large crack within the producing zone of the geologic formation, originating from and radiating out from the wellbore, serves to increase the effective wellbore radius. The end result is that the producing well behaves as if the entire wellbore radius were increased significantly.
The hydraulic fracturing process involves targeting a portion of the strata surrounding the wellbore and injecting a specialized fluid into the wellbore at pressures sufficient to initiate and extend a fracture into the formation. The fluid which is injected through the wellbore typically exits through holes which are formed in the cemented well casing using a special tool known as a perforating gun. However, sometimes wells are completed with no casing and therefore no perforations exist so that fluid is injected through the wellbore and directly to the formation face. Whether the well is cased or uncased, what is usually created by this process is not a single fracture, but a fracture zone, i.e., a zone having multiple fractures, or cracks in the formation, through which hydrocarbon fluids can flow to the wellbore and be produced at the surface. These fractures are extended by continued pumping and are either propped open with sand or other propping agents, or the fracture faces are etched by a reactive fluid such as an acid, or both. These techniques allow hydrocarbons contained in the formation to more readily flow to the fractures to the well bore. The artificially created fractures may be complimented by naturally existing fractures, or by fractures caused by previous or simultaneous stimulation operations in the same or nearby formations. The quality of the fracturing operation obviously has a great effect on the overall success or failure of the well production.
When fractures are created from a substantially vertical well bore penetrating the formation, there are often only two vertical fracture wings which are produced. Because these conditions have generally been viewed as less than optimum for hydrocarbon production, techniques have been developed to maximize the number of fractures created in the subterranean formation in both vertical and deviated wellbores. Because a larger number of fractures are being created, the interval or distance being stimulated is also generally increased. For example, U.S. Pat. No. 3,835,928 discloses a method of forming a plurality of vertically disposed spaced fractures from a deviated well bore penetrating a formation. A deviated well bore is drilled in a direction transverse to a known preferred fracture orientation and spaced fracture initiation points are created in the deviated well bore. Spaced vertical fractures are produced in the formation by separately creating and extending a fracture from each fracture initiation point.
U.S. Pat. No. 4,850,431 has as its object to create a plurality of spaced, substantially parallel fractures from a deviated wellbore. The in situ least principal stress direction of the formation is first determined. A predetermined number and size of perforations are then created in the casing at spaced fracture initiation points. In the preferred technique, each set of perforations is isolated and hydraulic pressure is applied to open the perforations and initiate fracturing.
One problem with the prior art fracturing techniques which Applicant's invention is intended to address is based partly upon the realization that increasing the number of fractures available to accept fracturing fluid and/or increasing the distance or interval being treated might actually work at cross purposes to the stated objective of achieving the greatest degree of hydrocarbon production. This can be explained, at least in part, because a greater number of fractures over a larger formation distance provides an increased possibility that all or most of the fracturing fluid will enter only the first or first few perforated intervals rather than being spread evenly across all the desired perforated intervals.
One deficiency in the prior art techniques therefore involves the type of perforating technique employed. The previously described references and others teach techniques for creating, for example, three or more perforated intervals in a given wellbore, each perforated interval having a given predetermined perforation shot count. In the charge carrier of a conventional perforating gun, the charges are spaced at, for example, a 60 degrees phasing and at a vertical distance of about 2 inches. Such a conventional configuration results in a shot density of 6 shots per foot using a 3⅜ inch gun in a 5½ inch casing. To achieve a higher shot count, for example 60 holes, the perforation interval would have to be on the order of 10 feet. A number of references in the perforating gun arts are directed to methods and apparatus for maximizing the number and size of holes created in the well casing which serve as fracture initiation points. However, none of these references, to Applicant's knowledge, teach the advantage of limiting the formation distance or interval being shot.
U.S. Pat. No. 5,323,684 shows an explosive carrier in which the explosive charges are mounted in a unique staggered spiral pattern which allows a greater number of shots that can be fired per unit length while increasing the spacing between explosive charges. The increased spacing of the charges is said to reduce the potential interference between fired shots, thereby providing a greater perforated hole size. However, specialized charge arrangements while achieving a greater shot density, sometimes fail to penetrate as deeply into the surrounding formation as compared to traditional off the shelf guns.
Despite the advances which have been made in the perforating and fracturing technologies of the type described above, a need continues to exist for further improvements which will result in even greater hydrocarbon production.
A need exists for improved techniques which will better insure that the fracturing fluid being pumped will flow more evenly through each set of perforations upon the application of hydraulic pressure, rather than the majority of the fluid entering only the first perforated interval of the wellbore.
A need exists for an improved fracturing technique which allows a predetermined target flow rate to be achieved early on in the pumping operation which flow rate creates a desired backpressure at the perforated intervals in the wellbore, whereby the fracturing fluid more evenly penetrates each perforated interval of the wellbore.